Devices for controlling the spatial distribution of the intensity, frequency, phase and/or polarization of electromagnetic waves, such as light, are often designated as spatial light modulators (SLM's). Such devices, which can be used in processing data, are capable of spatially modulating parallel discrete portions, or pixels, of a collimated coherent or incoherent beam of light with, for example, input data which is to be processed. The devices can be appropriately coupled to optical data processing systems, for example, into which the light beam spatially modulated with parallel input data is supplied at a rate commensurate with the processing system's potential throughput, the optical processing system utilizing parallel processing of such data without the limitations normally imposed by the need for serial manipulation of the data.
Many different forms of spatial light modulators have been suggested by those in the art. An early article, entitled "Spatial Light Modulators", by David Casasent and published in the Proceedings of the IEEE, Vol. 65, No. 1, January 1977, at pages 143-157, provides a summary of various types of spatial light modulators that have been suggested by the art. The devices described therein include SLM's using liquid crystal materials; materials which undergo surface deformation patterns (sometimes referred to as deformable SLM's), i.e., thermoplastic materials, dielectric oils, ruticon, or elastomers, or membranes combined with surface channel charged coupled devices (CCD's); alkali halide materials having intentionally introduced color center defects (sometimes referred to as photodichroic SLM's); materials which exhibit the Pockels effect (sometimes referred to as Pockels SLM's); materials using ferroelectric-photoconductor characteristics; materials using ferroelectric-photorefractive characteristics; and SLM's using acousto-optic techniques, magneto-optic techniques, techniques utilizing the characteristics of amorphous semiconductor materials; and techniques using magnetic-bubble devices. A later published article, entitled "A review of Spatial Light Modulation", by A. D. Fisher, presented at the Topical Meeting on Optical Computing, sponsored by the Optical Society of America, at Incline Village, Nev., Mar. 18-20, 1985, briefly discusses various SLM devices and the general status of the art at that time.
The devices discussed in the art can be either optically addressable or electrically addressable. Devices which are electrically addressable, such as devices which use membranes deflected by electrical signals supplied through electrodes in contact with the membrane or which use membranes combined with charged coupled devices, are relatively difficult to fabricate and the response is relatively slow so that such devices are not readily usable for high speed, real-time processing operations.
A spatial light modulator which operates in real time and which is primarily, and often preferably, electrically addressable or which may, alternatively, be optically addressable, has been disclosed in U.S. Pat. No. 4,696,533, issued on Sept. 29, 1987 to R. H. Kingston and F. Leonberger. The spatial light modulator disclosed therein is a relatively compact device handling a relatively large amount of input data in a relatively small volume, the device being capable of operation at high speeds, using up to as high as 10.sup.9 data samples per second. The device utilizes a suitable semiconductor substrate having a "buried channel" charge-coupled device (CCD) formed at a surface of the substrate. The amount of charge in the CCD channel material interaction regions associated with a plurality of light-transparent electrodes of a buried channel CCD is controlled by an electrical or optical data signal. In the specific embodiment disclosed therein, for example, the level of charge in such channel interaction regions thereby controls the electric field beneath the electrodes such that the light transmitted through the device is spatially modulated by the charge levels in such interaction regions in accordance with an electro-absorption effect, e.g., a Franz-Keldysch effect in a particular embodiment thereof.
Another semiconductor optical modulator has been described in the art, e.g., in the article "High-speed Optical Modulation with GaAs/AlGaAs Quantum Wells in a p-i-n Diode Structure" by T. H. Wood et al., Applied Physics Letters, Vol. 44, page 16, January 1984, which modulator uses a multiple quantum well (MQW) structure for producing a relatively stronger electro-absorption effect than that achievable using a Franz-Keldysch effect.
Still another device has been proposed which makes use of a structure containing a multiple quantum well (MQW) structure with a buried channel CCD fabricated on top of it. A pattern of charge packets is stored in the CCD and the magnitude of each charge packet controls the electric field across the corresponding underlying portion of the MQW structure. This in turn controls the optical transmission of that portion of the MQW structure. By applying a collimated beam of light to the pattern, one can provide for a spatial modulation of a one-dimensional or a two-dimensional input light beam. Such a device has been described, for example, in U.S. patent application, Ser. No. 050,197, filed on May 14, 1987 by B. E. Burke et al.
In such devices, the incoming light which is to be spatially modulated can be applied orthogonally to a surface, or input plane, of the device, the light passing through the device orthogonally to the interaction regions thereof (i.e., either the CCD channel material or the MQW material). The spatially modulated light thereupon exits either orthogonally from the opposite surface thereof or is reflected at such opposite surface so as to exit orthogonally therefrom at the same input plane to which the light was originally applied. The contrast that can be achieved in the modulated output light is relatively limited, however, since the modulation interaction regions thereof are relatively thin and the distance through which the incoming light travels orthogonally therein is relatively short. If such distance were longer, a greater contrast could be obtained.
A somewhat larger interaction path might be achieved merely by increasing the thickness of such interaction regions. However, when such thickness is increased, greater voltages must be used in order to achieve the required electrical field intensities. It has been found that increasing such voltages beyond a reasonable limit tends to cause a severe deterioration in the performance of the electrical addressing circuitry. Accordingly, there is a limit as to the amount by which the thickness of the interaction regions can be increased before undesirable performance degradation occurs.
In order to achieve a longer interaction path for the incoming light, it has also been proposed, for example, that the incoming light be applied to a cleaved facet and then guided through the interaction region in a direction substantially parallel, rather than orthogonal, thereto. The light then exits at the opposite facet with greater contrast for the modulated content thereof. However, it is not possible to use such latter technique for a two-dimensional array of light since the facets of the substrate can accommodate only a single line, or row, of incoming light.
It is desirable to provide a spatial light modulator which can produce greater contrast in the modulated output light while at the same time can be capable of responding to and spatially modulating a two-dimensional array of incoming light.